1. Field of Invention
The present invention relates to a virtual metrology (VM) method. More particularly, the present invention relates to a dual-phase virtual metrology method having promptness and accuracy.
2. Description of Related Art
In the semiconductor industry, run-to-run (R2R) advanced process control (APC) is widely applied to semiconductor and TFT-LCD factories for improving process capability. As defined in SEMI E133 specification, R2R control is the technique of modifying recipe parameters or the selection of control parameters between runs to improve processing performance. A run can be a batch, a lot, or an individual workpiece. A workpiece may represent a wafer for the semiconductor industry or a glass for the TFT-LCD industry. When L2L control is applied, only a single workpiece in the lot is required to be measured for feedback and feedforward control purposes. However, as device dimension shrinks further, tighter process control is needed. In this case, L2L control may not be accurate enough and workpiece-to-workpiece (W2W) control becomes essential for critical stages. As such, each workpiece in the lot should be measured. To accomplish this requirement, large amounts of metrology tools will be required and production cycle time will also be increased significantly. Furthermore, metrology delays, which are inevitable when real measurements are performed, will cause complicated control problems and degrade the APC performance.
Hence, there is a need to provide a virtual metrology method for providing a (virtual) measurement value with respect to each workpiece without actually measuring each workpiece, thereby performing W2W control, for example, by still using the measurement frequency of L2L control, i.e. only one single workpiece in the entire lot is spot-tested. However, generally speaking, both promptness and accuracy have to be taken into account when the virtual metrology method is used to feed virtual metrology (VM) values into W2W control, or the aforementioned requirements of W2W control cannot be met.